1. Field of the Invention
The present invention concerns methods and devices for monitoring an examination subject in a magnetic resonance apparatus.
2. Description of the Prior Art
Identification of the local SAR in an MR scanner has not previously been possible by means of measurement in clinical practice. The only known and used technique is to make use of a theoretical simulation (model) that accounts for both the patient and the structure of the transmission antenna as an electrical model. In the operation of MR scanners with a transmission array and with RF pulses that can exhibit an arbitrary pulse shape for each array element (variation of amplitude and phase), a multitude of overlay possibilities result. Comprehensive monitoring in the local SAR thus would involve a high degree of complexity, but monitoring of the local SAR value is absolutely necessary for the safety of the patient and is required by corresponding regulations. The overlaying of the electrical fields in array antennas is in particular critical because the E-vectors add linearly but the local power transfer is proportional to E2.
In a system with K elements, L phase steps and M amplitudes, (M*L)K combination possibilities result for every model point in the search for a maximum potential hotspot.
Given a typical exposed mass of 50 kg and a hotspot size of one gram, 50,000 model points result for which these combination possibilities must be considered. Given a higher channel count and appropriate resolution (phase and amplitude), the determination of all combinations for every model point is not applicable for clinical application. One possibility loophole is focusing on a reduced number of suitably selected model points that cover the possible amplitude and phase combinations as well as possible.
However, the monitoring for individually selected potential “hotspots” is generally not simple. For example, if a model point at which the E-fields of the individual antennas can theoretically superimpose at maximum (for example given antenna currents that are the same in terms of magnitude) is determined from among all model points in the patient model, this maximum is the case at this model point only for a specific distribution of the phases of these currents.
If the heat production determined by calculation at this model point is now monitored as being representative of all voxels, the actual maximum superimposition of the electrical fields can occur at other model points (that are not monitored) if the phase distribution of the currents deviates from the specific distribution (for example polarity reversal of a single antenna current). The SAR monitoring thus would be virtually blind to the actual occurring maximum.